The present invention relates to machine spindles and more particularly to high-speed drilling and routing spindles for fabricating printed circuit boards.
The production of printed circuit boards requires drilling large numbers of small-diameter holes in sheets of glass-reinforced epoxy and the like. The outside contours or edges of the boards are typically produced by routing. In order to obtain clean hole and edge geometry and high production rates, very high-speed rotation of the drill bits and/or routing cutters is required.
A numerically controlled circuit board drilling/routing machine is manufactured by Excellon Automation of Torrance, California. The machine includes a plurality of high speed spindle units, each unit of a typical model having a 1 horsepower, 60,000 rpm motor and a pneumatically operated collet mechanism for automatic exchange of drill bits and/or routing cutters. These spindle units are mounted vertically in a common frame above corresponding stacks of blank boards, the frame being capable of rapid raising and lowering for feeding the drill bits. An electronic control for the motors provides three-phase synchronous speed control and quick starting and stopping of the spindles. The machine provides automatic tool changes by (1) halting spindle rotation and positioning a tool receptacle under the collet; (2) opening the collet for releasing a first tool; (3) inserting a second tool in the collet; (4) closing the collet; and (5) resuming rotation of the spindle.
The spindle units of these machines exhibit a number of problems, including one or more of the following:
1. The collet mechanism has lateral play, causing the spindle to be unbalanced. When tool bits are changed, the mechanism can shift laterally, so that even if the spindle was balanced initially, it goes out of balance. This condition is greatly exaggerated when the collet is changed, so that severe damage results from inadvertent operation of the spindle without a collet.
2. The spindle is subject to harmonic resonance, especially at certain speeds and when subjected to lateral loads in routing operations. The associated resonant vibrations produce excessive wear of the spindle bearings and collet mechanism, contributing to an unbalanced condition of the spindle.
3. The collet mechanism incorporates a drawbar having a male hexagonal member slidingly engaging a corresponding female portion of the spindle shaft for preventing rotation of the drawbar with respect to the shaft. The engagement surfaces are subject to excessive wear because the high pressure angle of engagement, about 60.degree., produces very high compressive stresses at the points of engagement.
4. A compression spring is used between the collet and an actuating drawbar of the collet mechanism for preventing rotational adjustment of the collet when changing tool bits. The spring chatters in the routing mode, eroding away the drawbar and causing it to break.
5. The drawbar is biased axially by a plurality of belleville washers, the washers being located radially within a thin cylindrical cup fastened to the spindle shaft. The clearance between a cup that is required for expansion of the washers under axial compression permits the washers to shift radially, contacting one side of the cup and creating an unbalanced condition. When the washers are subsequently compressed for releasing the collet, they dig into the cup, deforming it. After repeated tool changes, there is substantial deformation of the cup, worsening the unbalanced condition of the spindle.
6. Rapid vertical movement of the frame produces axial movement of the drawbar within the spindle, especially when there is no tool in the collet, and when the collet is removed, resulting in harmful contact between the drawbar and a non-rotating actuating plunger. If the spindle is rotating when this happens, the result is destructive wear of the drawbar and the plunger.
7. The spindle bearings are axially preloaded by a wavy spring washer. The rapid vertical movement of the spindle and/or axial shocks associated with operation of the collet mechanism cause this washer to collapse and become permanently deformed. Thus the desired preloading is lost.
8. It is impractical to balance the spindle shaft together with its bearings and associated parts. Thus the shaft, with its armature laminations alone are balanced. The associated parts, including clamp nuts for the bearings are often significantly out of balance in spite of being machined precisely concentric because the steel alloys used in these parts have uneven density.
9. The machine is not reliably interlocked to prevent the independent operation of the spindle units without the cutting tool and/or collet in place.
10. Fiberglas drillings and dust particles get into the spindle housing through a labyrinth seal, eroding the seal and damaging the lower bearing of the spindle.
A large number of these machines are in use, involving great expenditures of money in the repair and replacement of the prematurely failing spindle units. Thus there is a need for a high-speed spindle assembly for drilling and/or routing printed circuit boards that avoids the above problems and is easy and inexpensive to produce. In addition, there is a need for a method for correspondingly improving existing spindle units.